This work has been financially supported by NSERC, CSEE, nanoBridge and TRLabs.

As mentioned above, the two key steps in the AAO-based fabrication process are (a) synthesis of the empty AAO template on arbitrary (primarily conductive and/or transparent) substrates (schematic description in Figure 1) and (b) growth of small molecular organic nanowires within the nanopores of the AAO template (Figure 2). In this section we provide a detailed description of these processes.
1. Growth of Anodic Aluminum Oxide (AAO) Templates on Conductive Aluminum Substrates
<ol>
	<li>Create nanoporous alumina templates by first polishing aluminum foils and then anodizing (or electrochemically oxidizing) them. Begin by cutting out small (~2 x 2 cm2) sheets of high purity unpolished aluminum (99.997%) with thickness 250 &mu;m.</li>
	<li>In lieu of electropolishing, a simpler chemical polishing process 18 is used. Submerge a small number of 2 x 2 cm2 sheets in nitric-phosphoric acid etchant at 80 &deg;C on a hotplate for 5 min.</li>
</ol>
Note: The nitric-phosphoric acid solution used to pre-treat the aluminum sheets is 15 parts 68% nitric acid and 85 parts 85% phosphoric acid. A polishing step is necessary prior to anodization because surface roughness of as-purchased aluminum is of the order of few microns, which creates a non-uniform electric field at the surface and prevents formation of an ordered pore array. In literature, electropolishing has been used extensively for this purpose 2,3. However, chemical polishing is a cheap and easy alternative, which also yields polished surfaces with comparable (or better) smoothness 18.
<ol start="3">
	<li>After etching, neutralize the foils in 1 M sodium hydroxide for 20 min. These "chemically polished" foils are now ready to be anodized.</li>
	<li>Load the polished aluminum sheets in to flat cells and anodize them for 15 min with 3% oxalic acid at 40 V DC bias.</li>
</ol>
Note: For foil samples a two-step anodization process is performed to improve pore ordering 2,3,19. This first step will create a porous oxide layer on the Al surface and nano-scale dimples at the aluminum/alumina interface, which act as the initiation sites for pore growth during the second step of anodization.
<ol start="5">
	<li>Etch the sample in chromic-phosphoric acid by removing it from the flat cell. Immerse the sample in a beaker of the etchant on a hot plate at 60 &deg;C for ~30 min to remove the initial oxide layer.</li>
	<li>Repeat the anodization process (step 1.4) for 2.5 min while keeping all other parameters unchanged. Try to realign the foil in the flat cell such that the same area anodized in step (1.4) will again be exposed to the electrolyte.</li>
</ol>
Note: The time of the final anodization step determines the thickness of the final oxide layer and can be altered accordingly. Duration of 2.5 min. corresponds to a film thickness (pore length) of ~500 nm. At the end of the second step a well-ordered nanopore array is created in the anodic alumina layer. The anodization and etching cycle can be repeated to further improve pore ordering.
<ol start="7">
	<li>Submerge the template in 5% phosphoric acid at room temperature for 40 min to thin the barrier layer at the bottom of the nanopores and widen the nanopore diameter. Final nanopore diameter after this step is ~60-70 nm.</li>
</ol>
2. Growth of AAO Templates on Transparent Substrates (Glass)
<ol>
	<li>Deposit the following multilayer system sequentially on cleaned glass slides: TiO2 (20 nm, atomic layer deposition), Au (7 nm, sputtering), Al (1 &mu;m, sputtering).</li>
</ol>
Note: The Au layer acts as an electrode, required for anodization, and does not deteriorate transparency 20. The TiO2 acts as a transparent adhesion layer between the Au and glass substrate.
<ol start="2">
	<li>Attach a foil electrode to the surface of the thin film of aluminum to be anodized using a conductive silver epoxy. This will result in a proper connection from power source to sample while improving current distribution.</li>
</ol>
Note: Since there is very little aluminum deposited on the glass substrate, polishing techniques mentioned before are not viable to flatten the aluminum surface. Instead, this protocol modifies the anodization procedure to incorporate another anodization/etching step.
<ol start="3">
	<li>Load the sample in to the flat cell and anodize the aluminum thin film for 4 min using 3% oxalic acid at 30 V DC bias.</li>
	<li>Without removing the sample from the flat cell, rinse the cell out with DI water and etch the template in chromic-phosphoric acid at 60 &deg;C for 1 hr by pouring the hot etchant in to the cell.</li>
</ol>
Note: The temperature of the etchant will immediately start to decrease once it has been poured in to the cell. Therefore, the duration of etching is increased to 1 hr from 30 min for the foil samples to ensure all oxidized film is removed.
<ol start="5">
	<li>Rinse out the cell again and anodize a second time under the same conditions as the first; for 4 min, using 3% oxalic acid at 30 V DC bias.</li>
	<li>Repeat step (2.4). Without removing the sample from the flat cell, rinse the cell out with DI water and etch the template in chromic-phosphoric acid at 60 &deg;C for 1 hr by pouring the hot etchant in to the cell.</li>
	<li>Rinse the cell out for the final time and perform the third (and last) anodization step, using 3% oxalic acid at 30 V DC. Monitor the current of the system to determine when to stop.</li>
</ol>
Note: The current needs to be monitored during the final anodization. After the first few seconds, the current stabilizes at around 1-2 mA. This indicates uniform anodization is taking place. Once the anodization process has consumed the remaining aluminum, the electrolyte solution (3% oxalic acid) will come in to contact with the underlying gold layer, which will cause a sharp increase in the anodization current (Figure 3). At this point, the anodization is stopped. The time should be roughly around the 4 min mark. This rise in current is not observed in foil samples (Figure 3) because a uniform barrier layer separates the solution and metal substrate.
<ol start="8">
	<li>Perform a pore widening step, similar to the foil sample protocol by submerging the template in 5% phosphoric acid at room temperature for 40 min.</li>
</ol>
Note: This will widen the pores but since the anodization process has eaten through the barrier layer there is none left to thin. Figure 4 shows the layered structure of glass substrate / 20 nm TiO2 / 7 nm Au / 500 nm porous Al2O3 with the absence of a barrier layer and pores clearly exposed to underlying Au thin film. Figure 5a and 5b shows empty AAO templates on foil and glass substrates respectively.
3. Centrifuge Assisted Growth of Small Molecular Organic Nanowires Within the Pores of AAO Template
<ol>
	<li>Prepare a saturated solution of the small molecular organic in a suitable solvent.</li>
</ol>
Note: The following organic molecules and solvents have been used: rubrene in acetone, Alq3 in chloroform and PCBM in toluene. From here on PCBM is referred to as the molecule of interest.
<ol start="2">
	<li>Load the templates in to the bottom of a centrifuge test tube such that the anodized area is facing the top of the test tube. The tube must be large enough to fit the sample inside.</li>
</ol>
Note: For foil samples, it is helpful to use a wafer of similar size to support the foil and prevent bending during centrifugation as described below. Figure 2 shows a schematic description of how the sample is mounted in the centrifuge.
<ol start="3">
	<li>Use a pipette to fill the test tube with enough PCBM solution such that the template is completely submerged.</li>
	<li>Load the test tube in the centrifuge and run for 5 min at 6,000 rpm.</li>
</ol>
Note: If the samples were mounted in the test tube at an angle, ensure the test tube is mounted in such a way that the anodized surface is pointing towards the center of the centrifuge (Figure 2).
<ol start="5">
	<li>Once the centrifuge has stopped, unload the test tubes and pour out the PCBM solution from the tubes.</li>
	<li>Remove the templates from the test tubes, or leave them at the bottom, for ~1 min to dry.</li>
	<li>Repeat steps 3.2-3.6 so that a total of 5-10 centrifuge runs have been performed.</li>
</ol>
Note: In situations where there is low solubility of the small molecule in its solvent, more centrifuge runs will help deposit more material in the nanopores.
<ol start="8">
	<li>Remove the sample from the bottom of the test tube and use a cotton swab soaked in toluene (or the respective solvent) to gently clean the surface of the template, removing any material that is left over on the surface of the template.</li>
</ol>

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D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Conjugated+Polymer+Photovoltaic+Cells.">Conjugated Polymer Photovoltaic Cells.</a> Chem. Mater. 16 (23), 4533-4542 (2004).</li></ol>

The authors declare that they have no competing financial interests.

Physical Picture for Nanowire Growth
It is first important to fully understand the growth method of the organic nanowires. Once we know exactly how they grow and form themselves in the pores we can use this deposition method to engineer nanostructures, devices and materials. In the past, polymer nanowires have been fabricated using the template wetting procedure without the assistance of a centrifuge, but for some materials such as organic small molecules, we have found this to be ineffective. D...

A template-assisted method is commonly used for the fabrication of vertically oriented nanowire arrays 1-3. This method allows straightforward fabrication of complex nanowire geometries such as an axially 4-6 or radially 7 heterostructured nanowire superlattice, which are often desirable in various electronic and optical applications. In addition, this is a low-cost, bottom-up nanosynthesis method with high throughput and versatility. As a result, template-directed methods have gained immense popularity among researchers worldwide 2,3.
The basic idea of the "template-directed method" is as follows. First a template is fabricated, which contains an array of vertically oriented cylindrical nanopores. Next, the desired material is deposited within the nanopores until the pores are filled. As a result the desired material adopts the pore morphology and forms a nanowire array hosted within the template. Finally, depending on the target application, the host template may be removed. However, this also destroys the vertical order. The geometry and dimensions of the final nanostructures mimic the pore morphology and hence synthesis of the host template is a critical part of the fabrication process.
Various types of nanoporous templates have been reported in literature 8. The most commonly used templates include (a) polymer track-etched membranes, (b) block copolymers and (c) anodic aluminum oxide (AAO) templates. To create the polymer track etched membranes a polymer foil is irradiated with high-energy ions, which completely penetrate the foil and leave latent ion tracks within the bulk foil 9. The tracks are then selectively etched to create nanosized channels within the polymer foil 9. The nanosized channels can be further widened by a suitable etching step. Key problems with this method are the non-uniformity of the nanochannels, lack of control of location, non-uniform relative distance between the channels, low density (number of channels per unit area ~108/cm2), and poorly ordered porous structure 1. In the block copolymer method a similar cylindrical nanoporous template is first created, followed by the growth of desired material within the pores 8.
In the past, methods (a) and (b) mentioned above have been used to fabricate polymer nanowires 8. However, these methods may not be suitable for synthesizing nanowires of any arbitrary organic material due to the potential absence of selective etching during post-processing steps. Post-processing typically involves removal of the host template, which for the above-mentioned templates would require organic solvents. Such solvents may have deleterious effect on the structural and physical properties of the organic nanowires. However, these templates work as ideal hosts for inorganic nanowires such as cobalt 10, nickel, copper and metallic multilayers 11, which remain unaffected in the etching process that removes the polymer host. Another potential challenge for the above-mentioned methods is the poor thermal stability of the host matrix at higher temperatures. High temperature annealing is often required to improve crystallinity of the organic nanowires, which indicates the necessity of good thermal stability of the host matrix.
Controlled electrochemical oxidation of aluminum (also known as "anodization" of aluminum) is a well-known industrial process and is commonly used in the automobile, cookware, aerospace and other industries to protect aluminum surface from corrosion 12. The nature of the oxidized aluminum (or "anodic alumina") depends critically on the pH of the electrolyte used for anodization. For corrosion-resistance applications, anodization is generally performed with weak acids (pH ~5-7), which create a compact, non-porous, "barrier-type" alumina film 12. However, if the electrolyte is strongly acidic (pH &lt; 4), the oxide becomes "porous" due to local dissolution of the oxide by the H+ ions. The local electric field across the oxide determines the local concentration of the H+ ions and hence surface pre-patterning prior to anodization offers some control over the final porous structure. The pores are cylindrical, with small diameter (~10-200 nm) and hence such nanoporous anodic alumina films have been used extensively in recent years for synthesizing nanowires of various materials 2,3.
Nanoporous anodic alumina templates offer better thermal stability, high pore density, long-range pore order, and excellent tunability of pore diameter, length, inter-pore separation and pore density via judicious choice of anodization parameters such as pH of the electrolyte and anodization voltage 2,3. Due to these reasons we choose AAO templates as the host matrix for the organic nanowire growth. Further, inorganic oxides such as alumina have high surface energy, thus facilitating uniform spreading of the organic solution (low surface energy) on the alumina surface 13. In addition, our goal is to grow these nanowire arrays directly on a conductive and/or transparent substrate. As a result, the pore is closed at the bottom end, which needs additional consideration as we describe below. Growth of nanowires within a through-pore template and subsequent transfer to the desired substrate is often undesirable due to poor interface quality and this method is not even feasible for short-length nanowires (or thin templates) due to poor mechanical stability of the thin templates.
&pi;-conjugated organic materials can be broadly classified into two categories: (a) long-chain conjugated polymers and (b) small molecular weight organic semiconductors. Many groups have reported synthesis of long chain polymer nanowires within the cylindrical nanopores of an AAO template in the past. Comprehensive review on this topic is available in refs 8,14. However, synthesis of nanowires of commercially important small molecular organics (such as rubrene, tris-8-hydroxyquinoline aluminum (Alq3), and PCBM) in AAO is extremely rare. Physical vapor deposition of rubrene and Alq3 within the nanopores of AAO template has been reported by several groups 4,15-17. However, only a thin layer (~30 nm) of organics can be deposited within the pores (~50 nm diameter) and prolonged deposition tends to block the pore entrance 4,16,17. Complete pore filling can be achieved in this method if the pore diameter is sufficiently large (~200 nm) 15. Thus it is important to find an alternative method that is applicable for pore diameters in the sub 100 nm range.
Another approach that has been used for some other small-molecular organics is a so-called "template wetting" method 8,14. However, in most reports thick commercial templates (~50 &mu;m) with both side open pores and large diameter (~200 nm) have been used. Such method has not produced nanowires in one-side closed pores as mentioned before, presumably due to the presence of trapped air pockets within the pores, which prevents infiltration of the solution within the pores. We have previously reported a novel method that overcomes these challenges and allows growth of small molecular organic nanowire arrays with arbitrary dimensions on any desired substrate. In what follows, we will describe the detailed protocol, potential limitations and future modifications.

<table><tbody><tr><td colspan="4">&lt;strong&gt;Reagents&lt;/strong&gt;</td></tr><tr><td>Toluene</td><td>Fisher Scientific</td><td>T324-4</td><td></td></tr><tr><td>68% Nitric Acid</td><td>Fisher Scientific</td><td>A200-212</td><td></td></tr><tr><td>85% Phosphoric Acid</td><td>Fisher Scientific</td><td>A242-4</td><td></td></tr><tr><td>10% Chromic Acid</td><td>RICCA Chemical Company</td><td>2077-32</td><td></td></tr><tr><td>10% Oxalic Acid</td><td>Alfa Aesar</td><td>FW.90.04</td><td></td></tr><tr><td>Chloroform</td><td>Fisher Scientific</td><td>C607-4</td><td></td></tr><tr><td>Aluminum Sheets</td><td>Alfa Aesar</td><td>7429-90-5</td><td></td></tr><tr><td>PCBM</td><td>Nano-C</td><td>Nano-CPCBM-BF</td><td></td></tr><tr><td>Alq3</td><td>Sigma Aldrich</td><td>444561-5G</td><td></td></tr><tr><td>Rubrene</td><td>Sigma Aldrich</td><td>551112-1G</td><td></td></tr><tr><td colspan="4">&lt;strong&gt;Equipment&lt;/strong&gt;</td></tr><tr><td>FlexAL Atomic Layer Deposition (ALD)</td><td>Oxford Instruments</td><td></td><td>For deposition of TiO2</td></tr><tr><td>PVD Sputter System</td><td>Kurt J. Lesker</td><td></td><td>For deposition of Au &amp;amp; Al</td></tr><tr><td>Flat Cell</td><td>Princeton Applied Research</td><td>K0235</td><td>For anodization of Al</td></tr><tr><td>Centrifuge</td><td>HERMLE Labnet</td><td>Z206 A</td><td>For deposition of organic nanowires</td></tr></tbody></table>

ultrahigh density array of vertically aligned small-molecular organic nanowires on arbitrary substrates

In recent years &pi;-conjugated organic semiconductors have emerged as the active material in a number of diverse applications including large-area, low-cost displays, photovoltaics, printable and flexible electronics and organic spin valves. Organics allow (a) low-cost, low-temperature processing and (b) molecular-level design of electronic, optical and spin transport characteristics. Such features are not readily available for mainstream inorganic semiconductors, which have enabled organics to carve a niche in the silicon-dominated electronics market. The first generation of organic-based devices has focused on thin film geometries, grown by physical vapor deposition or solution processing. However, it has been realized that organic nanostructures can be used to enhance performance of above-mentioned applications and significant effort has been invested in exploring methods for organic nanostructure fabrication.	
A particularly interesting class of organic nanostructures is the one in which vertically oriented organic nanowires, nanorods or nanotubes are organized in a well-regimented, high-density array. Such structures are highly versatile and are ideal morphological architectures for various applications such as chemical sensors, split-dipole nanoantennas, photovoltaic devices with radially heterostructured "core-shell" nanowires, and memory devices with a cross-point geometry. Such architecture is generally realized by a template-directed approach. In the past this method has been used to grow metal and inorganic semiconductor nanowire arrays. More recently &pi;-conjugated polymer nanowires have been grown within nanoporous templates. However, these approaches have had limited success in growing nanowires of technologically important &pi;-conjugated small molecular weight organics, such as tris-8-hydroxyquinoline aluminum (Alq3), rubrene and methanofullerenes, which are commonly used in diverse areas including organic displays, photovoltaics, thin film transistors and spintronics.	
Recently we have been able to address the above-mentioned issue by employing a novel "centrifugation-assisted" approach. This method therefore broadens the spectrum of organic materials that can be patterned in a vertically ordered nanowire array. Due to the technological importance of Alq3, rubrene and methanofullerenes, our method can be used to explore how the nanostructuring of these materials affects the performance of aforementioned organic devices. The purpose of this article is to describe the technical details of the above-mentioned protocol, demonstrate how this process can be extended to grow small-molecular organic nanowires on arbitrary substrates and finally, to discuss the critical steps, limitations, possible modifications, trouble-shooting and future applications.

As evidenced by the figures shown below (Figures 5 and 6), this centrifuge assisted drop casting method produces continuous nanowires. The nanowires, fabricated inside the pores of the AAO template, are vertically aligned, uniform, and electrically isolated from one another with capped bottoms. The diameter of the nanowires is determined by the diameter of pores in the template. They can be successfully fabricated on several different substrates, which lead to the potential application o...

Watch this Scientific Journal Video about Ultrahigh Density Array of Vertically Aligned Small-molecular Organic Nanowires on Arbitrary Substrates at JoVE.com

Ultrahigh Density Array of Vertically Aligned Small-molecular Organic Nanowires on Arbitrary Substrates

We report a simple method for fabricating an ultrahigh density array of vertically ordered small-molecular organic nanowires. This method allows for synthesis of complex heterostructured hybrid nanowire geometries, which can be inexpensively grown on arbitrary substrates. These structures have potential applications in organic electronics, optoelectronics, chemical sensing, photovoltaics and spintronics.

In  recent years &pi;-conjugated  organic semiconductors have emerged as the active material in a number of  diverse applications including large-area, ...

ultrahigh-density-array-vertically-aligned-small-molecular-organic

Research

JoVE Journal

Engineering

14.9K Views.  University of Alberta. We report a simple method for fabricating an ultrahigh density array of vertically ordered small-molecular organic nanowires. This method allows for synthesis of complex heterostructured hybrid nanowire geometries, which can be inexpensively grown on arbitrary substrates. These structures have potential applications in organic electronics, optoelectronics, chemical sensing, photovoltaics and spintronics.

Video: Ultrahigh Density Array of Vertically Aligned Small-molecular Organic Nanowires on Arbitrary Substrates

Fabricating Nanogaps by Nanoskiving

There are several methods of fabricating nanogaps with controlled spacings, but the precise control over the sub-nanometer spacing between two electrodes-and generating them in practical quantities-is still challenging. The preparation of nanogap electrodes using nanoskiving, which is a form of edge lithography, is a fast, simple and powerful technique. This method is an entirely mechanical process which does not include any photo- or electron-beam lithographic steps and does not require any special equipment or infrastructure such as clean rooms. Nanoskiving is used to fabricate electrically addressable nanogaps with control over all three dimensions; the smallest dimension of these structures is defined by the thickness of the sacrificial layer (Al or Ag) or self-assembled monolayers. These wires can be manually positioned by transporting them on drops of water and are directly electrically-addressable; no further lithography is required to connect them to an electrometer.

The fabrication of electrically addressable, high-aspect-ratio (> 1000:1) metal nanowires separated by gaps of single nanometers using either sacrificial layers of aluminum and silver or self-assembled monolayers as templates is described. These nanogap structures are fabricated without a clean room or any photo- or electron-beam lithographic processes by a form of edge lithography known as nanoskiving.

There are several methods of fabricating nanogaps with controlled spacings, but the precise control over the sub-nanometer spacing between two ...

Chemistry

Fabrication of Carbon Nanotube High-Frequency Nanoelectronic Biosensor for Sensing in High Ionic Strength Solutions

The unique electronic properties and high surface-to-volume ratios of single-walled carbon nanotubes (SWNT) and semiconductor nanowires (NW) 1-4 make them good candidates for high sensitivity biosensors. When a charged molecule binds to such a sensor surface, it alters the carrier density5 in the sensor, resulting in changes in its DC conductance. However, in an ionic solution a charged surface also attracts counter-ions from the solution, forming an electrical double layer (EDL). This EDL effectively screens off the charge, and in physiologically relevant conditions ~100 millimolar (mM), the characteristic charge screening length (Debye length) is less than a nanometer (nm). Thus, in high ionic strength solutions, charge based (DC) detection is fundamentally impeded6-8.
We overcome charge screening effects by detecting molecular dipoles rather than charges at high frequency, by operating carbon nanotube field effect transistors as high frequency mixers9-11. At high frequencies, the AC drive force can no longer overcome the solution drag and the ions in solution do not have sufficient time to form the EDL. Further, frequency mixing technique allows us to operate at frequencies high enough to overcome ionic screening, and yet detect the sensing signals at lower frequencies11-12. Also, the high transconductance of SWNT transistors provides an internal gain for the sensing signal, which obviates the need for external signal amplifier.
Here, we describe the protocol to (a) fabricate SWNT transistors, (b) functionalize biomolecules to the nanotube13, (c) design and stamp a poly-dimethylsiloxane (PDMS) micro-fluidic chamber14 onto the device, and (d) carry out high frequency sensing in different ionic strength solutions11.

We describe the device fabrication and measurement protocol for carbon nanotube based high frequency biosensors. The high frequency sensing technique mitigates the fundamental ionic (Debye) screening effect and allows nanotube biosensor to be operated in high ionic strength solutions where conventional electronic biosensors fail. Our technology provides a unique platform for point-of-care (POC) electronic biosensors operating in physiologically relevant conditions.

The unique electronic properties and high surface-to-volume ratios of single-walled carbon nanotubes (SWNT) and semiconductor nanowires (NW) 1-4 make them ...

Bioengineering

Well-aligned Vertically Oriented ZnO Nanorod Arrays and their Application in Inverted Small Molecule Solar Cells

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule (SMPV1) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), by utilizing ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The inverted SMPV1:PC71BM solar cells with ZnO NRs that grew on both a sputtered and sol-gel processed AZO seed layer are fabricated. Compared with the AZO thin film prepared by the sol-gel method, the sputtered AZO thin film exhibits better crystallization and lower surface roughness, according to X-ray diffraction (XRD) and atomic force microscope (AFM) measurements. The orientation of the ZnO NRs grown on a sputtered AZO seed layer shows better vertical alignment, which is beneficial for the deposition of the subsequent active layer, forming better surface morphologies. Generally, the surface morphology of the active layer mainly dominates the fill factor (FF) of the devices. Consequently, the well-aligned ZnO NRs can be used to improve the carrier collection of the active layer and to increase the FF of the solar cells. Moreover, as an anti-reflection structure, it can also be utilized to enhance the light harvesting of the absorption layer, with the power conversion efficiency (PCE) of solar cells reaching 6.01%, higher than the sol-gel based solar cells with an efficiency of 4.74%.

This manuscript describes how to design and fabricate efficient inverted SMPV1:PC71BM solar cells with ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The well-aligned vertically oriented ZnO NRs exhibit high crystalline properties. The power conversion efficiency of solar cells can reach 6.01%.

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule ...

Flow-assisted Dielectrophoresis: A Low Cost Method for the Fabrication of High Performance Solution-processable Nanowire Devices

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of nanowires. DEP is used for nanowire analysis, characterization, and for solution-based fabrication of semiconducting devices. The method works by applying an alternating electric field between metallic electrodes. The nanowire formulation is then dropped onto the electrodes which are on an inclined surface to create a flow of the formulation using gravity. The nanowires then align along the gradient of the electric field and in the direction of the liquid flow. The frequency of the field can be adjusted to select nanowires with superior conductivity and lower trap density. 
In this work, flow-assisted DEP is used to create nanowire field effect transistors. Flow-assisted DEP has several advantages: it allows selection of nanowire electrical properties; control of nanowire length; placement of nanowires in specific areas; control of orientation of nanowires; and control of nanowire density in the device. 
The technique can be expanded to many other applications such as gas sensors and microwave switches. The technique is efficient, quick, reproducible, and it uses a minimal amount of dilute solution making it ideal for the testing of novel nanomaterials. Wafer scale assembly of nanowire devices can also be achieved using this technique, allowing large numbers of samples for testing and large-area electronic applications.

In this paper, flow assisted dielectrophoresis is demonstrated for the self-assembly of nanowire devices. The fabrication of a silicon nanowire field effect transistor is shown as an example.

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of ...

Simultaneous Multi-surface Anodizations and Stair-like Reverse Biases Detachment of Anodic Aluminum Oxides in Sulfuric and Oxalic Acid Electrolyte

After reporting on the two-step anodization, nanoporous anodic aluminum oxides (AAOs) have been widely utilized in the versatile fields of fundamental sciences and industrial applications owing to their periodic arrangement of nanopores with relatively high aspect ratio. However, the techniques reported so far, which could be only valid for mono-surface anodization, show critical disadvantages, i.e., time-consuming as well as complicated procedures, requiring toxic chemicals, and wasting valuable natural resources. In this paper, we demonstrate a facile, efficient, and environmentally clean method to fabricate nanoporous AAOs in sulfuric and oxalic acid electrolytes, which can overcome the limitations that result from conventional AAO fabricating methods. First, plural AAOs are produced at one time through simultaneous multi-surfaces anodization (SMSA), indicating mass-producibility of the AAOs with comparable qualities. Second, those AAOs can be separated from the aluminum (Al) substrate by applying stair-like reverse biases (SRBs) in the same electrolyte used for the SMSAs, implying simplicity and green technological characteristics. Finally, a unit sequence consisting of the SMSAs sequentially combined with SRBs-based detachment can be applied repeatedly to the same Al substrate, which reinforces the advantages of this strategy and also guarantees the efficient usage of natural resources.

A protocol for fabricating nanoporous anodic aluminum oxides via simultaneous multi-surfaces anodization followed by stair-like reverse biases detachments is presented. It can be applied repeatedly to the same aluminum substrate, exhibiting a facile, high-yield, and environmentally clean strategy.

After reporting on the two-step anodization, nanoporous anodic aluminum oxides (AAOs) have been widely utilized in the versatile fields of fundamental ...

Synthesis of Substrate-Bound Au Nanowires Via an Active Surface Growth Mechanism

Advancing synthetic capabilities is important for the development of nanoscience and nanotechnology. The synthesis of nanowires has always been a challenge, as it requires asymmetric growth of symmetric crystals. Here, we report a distinctive synthesis of substrate-bound Au nanowires. This template-free synthesis employs thiolated ligands and substrate adsorption to achieve the continuous asymmetric deposition of Au in solution at ambient conditions. The thiolated ligand prevented the Au deposition on the exposed surface of the seeds, so the Au deposition only occurs at the interface between the Au seeds and the substrate. The side of the newly deposited Au nanowires is immediately covered with the thiolated ligand, while the bottom facing the substrate remains ligand-free and active for the next round of Au deposition. We further demonstrate that this Au nanowire growth can be induced on various substrates, and different thiolated ligands can be used to regulate the surface chemistry of the nanowires. The diameter of the nanowires can also be controlled with mixed ligands, in which another &#34;bad&#34; ligand could turn on the lateral growth. With the understanding of the mechanism, Au nanowire-based nanostructures can be designed and synthesized.

We report a solution-based method to synthesize substrate-bound Au nanowires. By tuning the molecular ligands used during the synthesis, the Au nanowires can be grown from various substrates with different surface properties. Au nanowire-based nanostructures can also be synthesized by adjusting the reaction parameters.

Advancing synthetic capabilities is important for the development of nanoscience and nanotechnology. The synthesis of nanowires has always been a ...

Multiscale Structures Aggregated by Imprinted Nanofibers for Functional Surfaces

Multiscale surface structures have attracted increasing interest owing to several potential applications in surface devices. However, an existing challenge in the field is the fabrication of hybrid micro-nano structures using a facile, cost-effective, and high-throughput method. To overcome these challenges, this paper proposes a protocol to fabricate multiscale structures using only an imprint process with an anodic aluminum oxide (AAO) filter and an evaporative self-aggregation process of nanofibers. Unlike previous attempts that have aimed to straighten nanofibers, we demonstrate a unique fabrication method for multiscale aggregated nanofibers with high aspect ratios. Furthermore, the surface morphology and wettability of these structures on various liquids were investigated to facilitate their use in multifunctional surfaces.

Presented is an easy method to fabricate nano-micro multiscale structures, for functional surfaces, by aggregating nanofibers fabricated using an anodic aluminum oxide filter.

Multiscale surface structures have attracted increasing interest owing to several potential applications in surface devices. However, an existing ...

Iron Nanowire Fabrication by Nano-Porous Anodized Aluminum and its Characterization

Magnetic nanowires possess unique properties that have attracted the interest of different fields of research, including basic physics, biomedicine, and data storage. We demonstrate a fabrication method for iron (Fe) nanowires&#160;via electrochemical deposition into anodic&#160;alumina oxide (AAO) templates. The templates are fabricated by anodization of aluminum (Al) discs, and the pore length and diameter are controlled by changing the anodizing conditions. Pores with an average diameter of around 120 nm are created using oxalic acid as the electrolyte. Using this method, cylindrical nanowires are synthesized, which are released by dissolving the alumina using a selective chemical etchant.

In this work, we describe a&#160;protocol to fabricate iron nanowires, including the formation of the porous alumina membrane that is used as the template, electrodeposition into templates using electrolyte solution, and the release of the nanowires into the solution.

Magnetic nanowires possess unique properties that have attracted the interest of different fields of research, including basic physics, biomedicine, and ...

DNA Origami-Mediated Substrate Nanopatterning of Inorganic Structures for Sensing Applications

Structural DNA nanotechnology provides a viable route for building from the bottom-up using DNA as construction material. The most common DNA nanofabrication technique is called DNA origami, and it allows high-throughput synthesis of accurate and highly versatile structures with nanometer-level precision. Here, it is shown how the spatial information of DNA origami can be transferred to metallic nanostructures by combining the bottom-up DNA origami with the conventionally used top-down lithography approaches. This allows fabrication of billions of tiny nanostructures in one step onto selected substrates. The method is demonstrated using bowtie DNA origami to create metallic bowtie-shaped antenna structures on silicon nitride or sapphire substrates. The method relies on the selective growth of a silicon oxide layer on top of the origami deposition substrate, thus resulting in a patterning mask for following lithographic steps. These nanostructure-equipped surfaces can be further used as molecular sensors (e.g., surface-enhanced Raman spectroscopy (SERS)) and in various other optical applications at the visible wavelength range owing to the small feature sizes (sub-10 nm). The technique can be extended to other materials through methodological modifications; therefore, the resulting optically active surfaces may find use in development of metamaterials and metasurfaces.

Here, we describe a protocol to create discrete and accurate inorganic nanostructures on substrates using DNA origami shapes as guiding templates. The method is demonstrated by creating plasmonic gold bowtie-shaped antennas on a transparent substrate (sapphire).

Structural DNA nanotechnology provides a viable route for building from the bottom-up using DNA as construction material. The most common DNA ...

Using Carbon Nanofiber Arrays for Efficient Biomolecule and Dye Delivery to Plants

Using Vertically Aligned Carbon Nanofiber Arrays on Rigid or Flexible Substrates for Delivery of Biomolecules and Dyes to Plants

The delivery of biomolecules and impermeable dyes to intact plants is a major challenge. Nanomaterials are up-and-coming tools for the delivery of DNA to plants. As exciting as these new tools are, they have yet to be widely applied. Nanomaterials fabricated on rigid substrate (backing) are particularly difficult to successfully apply to curved plant structures. This study describes the process for microfabricating vertically aligned carbon nanofiber arrays and transferring them from a rigid to a flexible substrate. We detail and demonstrate how these fibers (on either rigid or flexible substrates) can be used for transient transformation or dye (e.g., fluorescein) delivery to plants. We show how VACNFs can be transferred from rigid silicon substrate to a flexible SU-8 epoxy substrate to form flexible VACNF arrays. To overcome the hydrophobic nature of SU-8, fibers in the flexible film were coated with a thin silicon oxide layer (2-3 nm). To use these fibers for delivery to curved plant organs, we deposit a 1 &#181;L droplet of dye or DNA solution on the fiber side of VACNF films, wait 10 min, place the films on the plant organ and employ a swab with a rolling motion to drive fibers into plant cells. With this method, we have achieved dye and DNA delivery in plant organs with curved surfaces.

Author Spotlight: Unlocking Plant Transformation by Innovating with Carbon Nanofiber Arrays 

Here we describe methods for microfabricating vertically aligned carbon nanofibers (VACNFs), transferring VACNFs to flexible substrates, and applying VACNFs on both rigid and flexible substrates to plants for biomolecule and dye delivery.

The delivery of biomolecules and impermeable dyes to intact plants is a major challenge. Nanomaterials are up-and-coming tools for the delivery of DNA to ...